The new drug, Aimovig, is an antibody, just like the antibodies our immune system makes to attack viruses and other intruders. These antibodies have been produced in a lab, designed to glom onto receptors in our body that would otherwise interact with a protein called CGRP. It’s not clear exactly what CGRP does, or why levels increase during migraines. Amgen, Aimovig’s maker, sums up the state of current knowledge on its website: “There may be a connection between CGRP, CGRP receptors, and migraine.”

If your migraines are severe enough, the drug may still be worth it. Amgen thinks it can get the drug covered by most insurance for people with at least four migraines per month, and it’s offering to discount patients’ copays (they told investors there will be a $5 copay program) to encourage people to ask for the drug while still getting insurance companies to foot most of the bill. The Aimovig website also currently touts a two-month free trial for new patients.

It’s not a miracle drug, though. The Aimovig website trumpets that people with chronic migraines had six to seven fewer migraine days per month, but people who took a placebo in that same study (an injection without any drug) saw their migraine days go down as well. The FDA announcement gave the numbers as compared to the placebo group in each study, which is the right way to do it. In the three studies, people had an average of one fewer migraine day per month, one to two fewer days, or (for the people with the most frequent migraines) two and a half fewer.

Those are averages, though, and they hide something very significant: 40 percent of the migraine patients in the trials got a huge reduction in migraine days—50 percent or better. If you’re one of those people, great! And if you’re one of the 24 percent who had the same reduction in the placebo group, hey, I’m happy for you too. But for some people to have this strong reaction, others must have had no effect or a very small effect. So there’s a chance that some of the people who (get their insurance to) pay $575 a month for this drug might not get any benefit at all. So far, there’s no way to know before you try it which group you’ll fall into.